TOKYO â€” Beijing's assertiveness in the South China Sea could soon be replicated in neighbouring waters, a Japanese government-backed report said Friday, amid rising regional nervousness about China's intentions.

Tokyo should pay close attention to any indication that its giant neighbour is looking to spread its wings into the East China Sea, which abuts Japan, the National Institute for Defence Studies warned.

"For China, just like the South China Sea, the East China Sea is an important route for its advance into the oceans, and if China's military power improves . . . it is likely that China will adopt a similar assertive attitude towards this water area as shown in the South China Sea," it said.

"Therefore, more attention should be paid to the PLA's actions in the waters surrounding Japan," it said, referring to China's People's Liberation Army.

The China Security Report, funded by Japan's defence ministry, was published Friday for only the second time and comes against the backdrop of a rapid military build-up by Beijing that has set alarm bells ringing across Asia and in Washington.

China lays claim to essentially all of the South China Sea, where its professed ownership of the Spratly archipelago overlaps with claims by Vietnam, the Philippines, Taiwan, Brunei and Malaysia.

Beijing and Tokyo have a long-standing dispute over an uninhabited but strategically coveted island chain known as Senkaku in Japanese and Diaoyu in Chinese, which lies between Japan and Taiwan in the East China Sea.

The two sides have occasionally butted diplomatic heads over the issue, most notably in late 2010, when Japan arrested the captain of a Chinese fishing vessel near the island chain after a collision with its coastguard.

The skipper was held for more than two weeks as Tokyo and Beijing faced off, with relations becoming distinctly chilly before Japan released him, in what was widely seen as a climbdown.

That episode notwithstanding, China has generally shied away from direct confrontations with Japan in the area, the report said.

"Thus far, unlike in the South China Sea, China has not taken such provocative actions in the East China Sea as physical disturbances of foreign survey vessels and major live-fire naval exercise" because of worries about hurting ties with Japan and the United States, the report said.

But Beijing's drive to boost its naval reach and tip the military balance in the Pacific away from the United States may change this, the report said, noting the ongoing construction of a large-scale naval base on Hainan island.

China's military budget has seen double-digit increases every year for much of the last decade, but is hampered in its ability to become a "blue seas" power like the U.S. because its access to the Pacific is largely cut off by Japanese islands and their surrounding territorial waters.

Its navy has increasingly sailed between these islands as it reaches further afield, the Japanese report said.

The United States, which has tens of thousands of troops stationed in East Asia, has repeatedly stressed its commitment to the Pacific theatre, a stance welcomed by many of China's smaller neighbours.